This invention relates to a refrigerant tube of a flat type for circulating a refrigerant and to a heat exchanger using the refrigerant tube. Such a heat exchanger is particularly useful in an air-conditioning device for an automobile.
A heat exchanger is classified into various types such as a multiflow condenser, a serpentine heat exchanger, a heater core, and a radiator and uses a bank of refrigerant tube or tubes of a flat type for circulating a refrigerant.
The flat-type refrigerant tube generally comprises a flat aluminum plate having a given length. The aluminum plate is provided with a plurality of refrigerant passage chambers (hereinafter simply called chambers) extending therein along its longitudinal direction and arranged in parallel with one another in a plane parallel to the plate. Each of the chambers has a rectangular cross-section and is defined by a first pair of wall surfaces parallel with and opposite to each other and a second pair of wall surfaces parallel with and opposite to each other and perpendicular to the first pair of wail surfaces. The first pair of wall surfaces connect to the second pair of the wall surfaces to form four corners of the chamber.
The heat exchanger is required to have a high heat exchange efficiency. One of the factors to determine the heat exchange efficiency is a heat-transfer area of the refrigerant. Generally, a greater heat-transfer area achieves a higher heat exchange efficiency. Accordingly, the heat exchanger is required to have a large heat-transfer area of the refrigerant.
In the heat exchanger using the flat-type refrigerant tube, the heat-transfer area of the refrigerant corresponds to a total area of the first and the second pairs of the wall surfaces defining each of the chambers in the flat-type refrigerant tube.
In the flat-type refrigerant tube known in the art, each corner in each of the chambers is formed to have a right angle in order to increase the heat-transfer area of the refrigerant. Typically, the flat-type refrigerant tube is manufactured through an extrusion molding process using a die. Due to the restraint upon manufacture of the die itself, each corner actually has an inevitable small curvature R. The inevitable small curvature R is approximately equal to 0.05 mm.
Another approach to increase the heat-transfer area is additionally adopted in the prior art of the flat-type refrigerant tube. The approach is to make the flat-type refrigerant tube have elongated protrusions or ribs formed on at least one of the first and the second pairs of wall surfaces of each chamber and extending along a longitudinal direction thereof. For example, each of Japanese Design Registrations Nos. 624349-1 and 711576 discloses the flat-type refrigerant tube having those protrusions or ribs.
However, the conventional flat-type refrigerant tube with the corners having a right angle or an inevitable small curvature R is disadvantageous in that a pressure-resistant strength is low due to its configuration. When assembled into the heat exchanger and practically used, the flat-type refrigerant tube is subjected to a stress due to the pressure of the refrigerant. In this situation, the low pressure-resistant strength would cause a serious problem. In detail, the stress due to the pressure of the refrigerant tends to concentrate onto the corners. Thus, the corners can easily be damaged and therefore have a less durability. In order to solve the problem about the pressure-resistant strength in the conventional flat-type refrigerant tube, walls defining a plurality of the chambers are increased in thickness. However, the increase in thickness of the walls is disadvantageous because it results in deterioration of the heat exchange efficiency and increase of the weight of the flat-type refrigerant tube.